Valves for controlling the flow of gases can be subjected to extremely demanding performance expectations. In particular, on-off solenoid valves are expected to provide leak-tight sealing when closed and to respond quickly when between on and off. Challenges to such seemingly innocuous operation include contaminants which affect sealing, differential pressure across the valve which may vary widely, seal life and in the case of a valve for a gas cylinder, the avoidance of overfilling. Problems associated with sealing are exacerbated when at high pressures and with low molecular weight gases.
In more detail, while flow-limiting orifices in such systems can be 0.250″ (6.35 mm) in diameter and larger, prior art designs are expected to provide leak-tight sealing when closed, with the maximum allowable leak often, under some regulatory guidelines, being equivalent to a 5 μ-in hole. At the same time, contaminants routinely found in the fluid flow stream may be 10,000 times or more that that size. Such contaminants can come from various sources. In fuel gases as a typical example, contaminants may come from debris in lines at the fuel process plant; scale in the fuel distribution pipelines; debris from the installation of tubing and fittings in the application system's plumbing; and debris inside the storage cylinders left over from their original manufacture.
Currently available prior art systems do not address the issue of contaminants being present in the flow and thus are prone to erratic performance in the field.
On-off solenoid valves for high pressure operation are typically of the pilot-operated type. That is, a direct acting solenoid opens a small orifice, being typically 0.010″ to 0.030″ in diameter, which provides a small pilot flow. The pilot flow serves to charge the downstream system, slowly raising the pressure therein to the supply pressure. When the downstream pressure has nearly reached the supply pressure, a second stage of the solenoid is able to open, allowing the primary flow orifice, being typically 0.156″ to 0.250″ in diameter, to provide normal, full-flow rates. At high supply pressures and with smaller system volumes, the delay time from pilot flow to full flow is substantially indiscernible. However, at low supply pressures, for example less than 500 psig, the delay time becomes significant and can often reach 30 seconds or more. Such delays are not tolerable in many applications, such as automotive applications. Accordingly, it is desirable to have a system with little delay, regardless of supply pressure, downstream pressure and flow demand conditions.
As noted above, many applications for on-off solenoid valves are increasingly likely to demand leak-tight sealing across the system's entire pressure range, for example 7 to 350 bar. In some applications, the maximum allowable leak rate may be created by a leakage path equivalent to a 5 μ-in (0.127 micron) hole. Accordingly, conventional seal materials and configurations are often unable to reliably deliver the required performance. This is especially true for smaller molecule gases and higher operating pressures.
Such on-off valves are commonly required to have a manual override valve which can prevent both the filling and withdrawal of fluid from a cylinder, regardless of the solenoid's state. Currently, valves with rather common plastic seats and poorly matched operators are used in practice. The two most common types of valves have either overly large or seriously undersized operators. The oversized operators are bulky, expensive and can easily cause seal damage with seemingly reasonable torque levels. The undersized operators are themselves easily damaged and can have difficulty in transmitting enough torque to reliably open and close the valve. Further, conventional valves using prior art seals made from the PTFE class of materials may be prone to seal extrusion and failure at high differential pressures.
In some gaseous fast-fill applications, achieving a proper fill level can be difficult using conventional valves and seals. As the heat of compression briefly raises the in-cylinder pressure, the cylinder must be over-filled so that once equilibrium temperatures have been restored the cylinder will have the desired pressure. In a similar situation, the same gas may be stored at a variety of pressures, depending on the cylinder type and application. Preventing over-filling and over-pressurizing a cylinder is desirable.